The Kelvin (K) rating has nothing to do with the brightness of the light, it has to do with the color.
Plants like a K rating of around 6500. Usually I do planted tanks with one 6500 and one 10000 K light (I've got 2 for sale right now if they fit your fixture).
6500 encompasses more of the red-end of the color spectrum and I believe 10,000 encompasses the full spectrum and looks whiter.
Combining the two will bring out color in your fish and help keep your plants healthy. If your plants are healthy, they'll shade the tank, and your fish will be happy.